2005/01/17 by Hansjörg Geiges
Mathematics · #math.HO #math.SG #msc:01A45 #msc:53Dxx
published as Nieuw Arch. Wiskd. (5) 6 (2005), 117-123 · To appear in Nieuw Archief voor Wiskunde
arxiv created 2005/01/17 · arxiv updated 2009/12/01
This article is based on the author's inaugural lecture at the University of Cologne on 24 January 2003.